1. Field of the Invention
This invention relates to methods of treatment of blood vessels with bioabsorbable polymeric medical devices, in particular, stents.
2. Description of the State of the Art
This invention relates to radially expandable endoprostheses, that are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices that function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty, stenting, or valvuloplasty) with apparent success.
Stents are typically composed of scaffolding that includes a pattern or network of interconnecting structural elements or struts, formed from wires, tubes, or sheets of material rolled into a cylindrical shape. This scaffolding gets its name because it physically holds open and, if desired, expands the wall of the passageway. Typically, stents are capable of being compressed or crimped onto a catheter so that they can be delivered to and deployed at a treatment site.
Delivery includes inserting the stent through small lumens using a catheter and transporting it to the treatment site. Deployment includes expanding the stent to a larger diameter once it is at the desired location. Mechanical intervention with stents has reduced the rate of restenosis as compared to balloon angioplasty. Yet, restenosis remains a significant problem. When restenosis does occur in the stented segment, its treatment can be challenging, as clinical options are more limited than for those lesions that were treated solely with a balloon.
Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy uses medicated stents to locally administer a therapeutic substance. The therapeutic substance can also mitigate an adverse biological response to the presence of the stent. Effective concentrations at the treated site require systemic drug administration which often produces adverse or even toxic side effects. Local delivery is a preferred treatment method because it administers smaller total medication levels than systemic methods, but concentrates the drug at a specific site. Local delivery thus produces fewer side effects and achieves better results.
A medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier that includes an active or bioactive agent or drug. Polymeric scaffolding may also serve as a carrier of an active agent or drug by incorporating a drug through the scaffolding material.
The stent must be able to satisfy a number of mechanical requirements. The stent must be have sufficient radial strength so that it is capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel. This structural load will change as a function of time as the vessel heals, positively remodeling or adapting to the presence of the stent.
Once expanded, the stent must adequately provide lumen support during a time required for treatment in spite of the various forces that may come to bear on it, including the cyclic loading induced by the beating heart. In addition, the stent must possess sufficient flexibility with a certain resistance to fracture.
Stents made from biostable or non-erodible materials, such as metals, have become the standard of care for percutaneous coronary intervention (PCI) as well as in peripheral applications, such as the superficial femoral artery (SFA), since such stents have been shown to be capable of preventing early and later recoil and restenosis. For a variety of reasons, the performance of stents in the SFA appear to be more problematic than in coronary vessels and in other peripheral vascular beds, such as the iliac and carotid arteries. This may be due to the significant mechanical stresses placed on the devices in the dynamic SFA environment relative to other vasculature, as well as the vessel length and the severity of stenotic and occlusive disease. The SFA is subjected to various forces, such as compression, torsion, flexion, extension, and contraction, which place a high demand on the mechanical performance of implants.
However, a stent made out of such biostable material tends to retain mechanical or structural integrity and remains at the implant site indefinitely unless it is removed by intervention or is dislodged. Intervention presents risks to the patient and dislodgement can have significant adverse consequences on the patient. Leaving the stent at the implant site permanently also has disadvantages. One disadvantage is that the stented segment has the compliance of the stent which is very different from that of healthy vessel segment. Another drawback of such durably implanted stents is that the permanent interaction between the stent and surrounding tissue can pose a risk of endothelial dysfunction and late thrombosis.